Aromatic carbonate polymers are well known, commercially available materials having a variety of applications in the plastics art. Such carbonate polymers may be prepared by reacting a dihydric phenol, such as 2,2-bis(4-hydroxyphenyl)propane, with a carbonate precursor, such as phosgene, in the presence of an acid binding agent. See the Encyclopedia of Polymer Science and Technology, Vol. 10, pp. 710-764, Interscience, New York, 1969, which is incorporated herein by reference. Generally speaking, aromatic polycarbonate resins offer a high resistance to attack by mineral acids, and they are physiologically harmless as well as stain resistant. In addition, articles molded from such polymers have a high tensile strength and a high impact strength, except in thick sections, a high heat resistance, and a dimensional stability far surpassing that of most other thermoplastic material. However, in certain applications, the use of aromatic polycarbonate resins is limited because (i) they have a high viscosity in the melt, making molding of complex, large, and especially foamed parts difficult; (ii) they exhibit brittleness under sharp impact conditions in thick sections and regardless of thickness when small amounts of reinforcements, e.g., glass or pigments, e.g., titanium dioxide, are added for conventional purposes; and (iii) they exhibit severe environmental stress crazing and cracking. The term "environmental stress crazing and cracking" refers to the type of failure which is hastened by the presence of organic solvents, e.g., acetone, heptane, and carbon tatrachloride, when such solvents are in contact with stressed parts fabricated from aromatic polycarbonate resins. Such contacts may occur, for example, when the solvents are used to clean or degrease stressed parts fabricated from polycarbonates or when such parts are used in automobiles, especially under the hood, as well as in bumpers, which are often subjected not only to impact conditions over a wide temperature range but also subjected to contact with gasoline during refueling.
The relatively high melt viscosities and softening points of aromatic polycarbonates make them difficult to melt process, and, although several approaches have been suggested for improving melt flow, they have disadvantages. For example, plasticizers can be added but other important properties are lost, the parts becoming brittle and losing a substantial amount of their ability to resist distortion by heat. It is suggested in Goldblum, U.S. Pat. No. 3,341,224 that small amounts of polyethylene can be added. While this markedly enhances resistance to environmental stress cracking, low levels of polyethylene are not too effective to enhance melt flow and an increase into effective ranges tends to result in molded articles which delaminate. On the other hand, U.S. Pat. No. 4,088,771 to Gergen et al. suggests the admixing of from about 4 to about 96 parts by weight of a block copolymer and from about 4 to about 96 parts by weight of a polycarbonate so as to form at least partial continuous interlocking networks to provide compositions which exhibit good dimensional stability and integrity. While such compositions exhibit generally good properties where they contain a major proportion of block copolymer, that is, generally from 50 to 75% by weight or more, and even exhibit acceptable properties at the copolymer is reduced to 25% by weight, and even as low as 4% by weight, the advantageous properties thereof are generally reduced or adversely affected in one way or another as the amount of copolymer is reduced. This is especially so with respect to the room temperature impact strength thereof, as well as their resistance to delamination or deterioration when immersed in certain solvents or materials, such as gasoline, in compositions containing, for example, 4% by weight of the copolymer. Consequently, the utilization of such compositions which contain as little as 4% by weight of the copolymer is restricted to a certain extent. For example, shaped articles made of such compositions would have a limited useful life where used in articles subjected to repeated impact at room temperatures. In addition, the usefulness of such a composition in molded articles such as automobile bumpers or trim for automobile bumpers would also be somewhat restricted since such bumpers would be subject to impact at a wide variety of temperatures and, as well, splashing with gasoline, especially when refueling. There exists, therefore, the need for compositions such as those of the instant invention which do not exhibit the above mentioned disadvantages, and the compositions of the present invention fulfill this need.